Assessment of serotonin release in the living brain with positron emission tomography (PET) may have been hampered by the lack of suitable radioligands. We previously reported that fenfluramine caused a dose-dependent reduction in specific binding in monkeys using a classical displacement paradigm with bolus administration of [(11)C]AZ10419369. The aim of this study was to confirm our previous findings using an equilibrium approach in monkey. A total of 24 PET measurements were conducted using a bolus infusion protocol of [(11)C]AZ10419369 in three cynomolgus monkeys. Initial PET measurements were performed to assess suitable K(bol) values. The fenfluramine effect on [(11)C]AZ10419369 binding was evaluated in a displacement and pretreatment paradigm. The effect of fenfluramine on [(11)C]AZ10419369 binding potential (BP(ND)) was dose-dependent in the displacement paradigm and confirmed in the pretreatment paradigm. After pretreatment administration of fenfluramine (5.0 mg/kg), the mean BP(ND) of the occipital cortex decreased by 39%, from 1.38±0.04 to 0.84±0.09. This study confirms that the new 5-HT(1B) receptor radioligand [(11)C]AZ10419369 is sensitive to fenfluramine-induced changes in endogenous serotonin levels in vivo. The more advanced methodology is suitable for exploring the sensitivity limit to serotonin release as measured using [(11)C]AZ10419369 and PET.